In a wireless LAN system, it is required by a Federal Communications Commission (FCC) to monitor a presence of a radar system (The details of the requirement is described in FCC 06-96). For example, the radar system may be those used in the meteorological bureau. When a radar system is found in a communication band used by the wireless LAN system, the wireless LAN system is required to change the communication band to avoid a conflict with the radar system.
In a conventional wireless LAN system of Institute of Electrical and Electronics Engineers (IEEE) 802.11a, wireless communication is performed using a frequency band having 20 MHz bandwidth. Accordingly, a radar signal may be monitored for a frequency band having 20 MHz bandwidth. An example of such conventional technique is disclosed in JP-A-2005-210616 (counterpart U.S. publication is: US 2005/0162304 A1).
As a measure for increasing a transmission rate in a recent wireless LAN, there is proposed to extend the frequency band in IEEE 802.11n standard. In the IEEE 802.11n standard, wireless communication is also available in 40 MHz bandwidth in addition to the conventional 20 MHz bandwidth. When the conventional radar detecting method is employed to the system conforming to the 802.11n standard, the radar may be monitored in only one of the bandwidths. Therefore, when the radar is detected, the wireless LAN system needs to operate to either terminate the communication or change a communication channel, thereby deteriorating efficiency in usage of a frequency band.